Aluminum flake pigments are used extensively in coating compositions. Aluminum flake pigments are slightly reactive in an aqueous environment having a basic pH, such as aqueous coating compositions. The aluminum reacts with water to produce hydrogen gas and aluminum hydroxide. The formation of aluminum hydroxide occurs relatively rapidly with aluminum pigments due to the high surface to mass ratio of the small particles. The reaction is a form of corrosion and converts the pigment to a hydrated oxide form unsuitable for pigment use, as it destroys the metallic pigmentation properties of the mirror-like particles. The amount of corrosion is measured by the amount of hydrogen gas produced over a given period of time. Corrosion is exacerbated since the reaction of the aluminum in water results in the continuous formation of H.sup.+ and OH.sup.- ions. The H.sup.+ ions attack and corrode the aluminum and the OH.sup.30 ions cause the pH of the environment to further increase. In coating compositions, contact of the aluminum pigment with the environment is continuous over extended periods of time, since coatings containing the pigment are often stored for 6 months or more before application. Corrosion inhibition can be achieved if the rates of these reactions, or any partial steps involved, can be decreased.
It has been found that transition metal salts and rare earth metal salts inhibit corrosion in the aluminum flake pigment particles in a basic aqueous environment. It is thought that the metal salts inhibit corrosion due to a compact film of rare earth metal oxides and hydroxides that replace the natural aluminum oxide film on the surface of the aluminum flake pigment. It is hypothesized that the transition metal or rare earth metal oxide/hydroxide film forms at local cathodic sites on the surface, where the alkaline conditions generated by oxygen reduction reactions cause the aluminum oxide to dissolve and the transition metal or rare earth metal oxide to precipitate.
It has further been discovered that aluminum flake pigment treated with the transition metal and/or rare earth metal salts are highly effective to inhibit the corrosion of the pigment particles in water-borne coating compositions. This is significant because the water-borne coating compositions in use today have a basic pH. The pH of acrylic coating compositions typically ranges from 8.0-9.0, and the polyurethane coating compositions have a pH typically ranging from 7.5 to 8.0. The high pH of water-borne coating systems speeds up the reaction which produces the hydrated oxide aluminum and results in serious degradation or corrosion of aluminum flake pigment used in metallic coatings.
The pigments treated with transition metal and rare earth metal salts demonstrate excellent dispersibility in coatings. The use of the treated aluminum pigment does not result in any loss of adhesion to other layers or any cohesive failure within the coating layers. The coatings are particularly useful for automotive coating applications.